1. Field of the Invention
The present invention relates to methods of operating droplet deposition apparatus, in particular an inkjet print head, comprising a chamber communicating with a nozzle for ejection of ink droplets and with a supply of ink, the print head further comprising an electrically actuable device associated with the chamber and actuable a plurality of times to eject a corresponding number of droplets. In particular, it relates to a print head in which the chamber is a channel having associated with it a device arranged to vary the volume of the channel in response to an electrical signal.
2. Description of the Related Art
EP 0 422 870 discloses the concept of “multipulse grayscale printing”, i.e. firing a variable number of ink droplets from a single channel within a short period of time, the resulting “packet” of droplets merging in flight and/or on the paper to form a correspondingly variable-size printed dot in the paper. An inkjet print head incorporating this technique is now commercially available, e.g. the OmniDot 760/GS8 from Xaar (UK). The channels in this print head are separated one from the next by side walls which extend in the lengthwise direction of the channels. In response to electrical signals, the channel walls are displaceable transverse to the channel axis. This in turn generates acoustic waves that travel along the channel axis, causing droplet ejection from a nozzle located at one end of the channel, as well-known in the art. In EP 0 968 822 a method for driving a multipulse grayscale print head is disclosed. The driving method is based on the generation of a series of drive pulses applied to the electrodes of piezoelectric channel walls. A first voltage pulse deforms the piezoelectric channel walls so as to increase the volume of the ink chamber and create a negative pressure in the ink chamber; a subsequent voltage pulse decreases the volume of the ink chamber and increases the pressure in the ink chamber thereby ejecting a droplet from the ink chamber. The voltage pulses are then removed from the electrodes to bring the ink chamber back to its original volume. This sequence of drive pulses may be repeated a number of times corresponding to the number of droplets to be ejected successively for merging into a single variable-size drop. This sequence of drive pulses is often referred to as a waveform for generating a variable-size drop.
Multipulse grayscale print heads, also referred to as multidroplet print heads or simply grayscale print heads, are appreciated for their high print quality using the ‘variable-size drop’ feature. A drawback of multipulse grayscale print heads is lack of drop speed uniformity of the variable-size drops. It is for example known that the first droplet ejected from the print head is slower than successive droplets ejected within the same packet, i.e. within the same drop. This may be advantageous towards merging of subsequent droplets into the first droplet, but it is a disadvantage if the first droplet is printed on its own as a single drop. In other words, the average velocity of a single droplet drop is often lower than the average velocity of a multiple droplet drop. As a difference in average velocity results in a difference in flight time from the print head to a receiving medium, a single droplet drop usually hits the receiving medium at a later instance than a multiple droplet drop ejected at the same time. In inkjet printing applications, a relative movement between an inkjet print head and a receiving medium enables the printing of dots at predefined locations (raster or grid points) on the receiving medium. With this relative movement, a different landing time on the receiving medium therefore results in undesired dot placement variations from the ideal raster point location. This often limits the use of multipulse grayscale print heads to printing speeds below 0.5 m/s. The printing speed is the relative velocity between the receiving medium and the multipulse grayscale print head during printing.
In the prior art, different solutions have been proposed to equalize the average velocity of multipulse grayscale drops. A solution disclosed in U.S. Pat. No. 6,402,282 is to introduce an additional time delay between the application of successive drive signals generating successive droplets from a given channel. The time delay is chosen such that a variation in the average velocity at which the corresponding droplets travel to the receiving medium remains below a given value. The time delay is referred to as the channel dwell time. Unfortunately, adding time delays to a sequence of drive pulses reduces the maximum printing speed of the print head.
Another approach includes the application of a boost pulse prior to the application of the drive signal generating the first droplet. This boost pulse inputs an amount of energy in the ink chamber, prior and in addition to the energy provided through the drive signal of the first droplet. The additional energy input increases the energy available in the ink chamber for ejecting the first droplet and also increases the average velocity of the first droplet when ejected. The boost pulse is only applied prior to the drive signal for ejecting the first droplet and therefore does not affect successive droplets. So the velocity of the first droplet is increased while the velocity of successive droplets is theoretically maintained. In practice however, there remains a difference between the velocity of the first droplet and that of successive droplets, which make this approach not suitable for high speed printing applications. The application of a boost pulse prior to the main ejection pulse is disclosed in U.S. Pat. No. 6,857,715 and U.S. Pat. No. 6,231,151.
In patent application WO 98/08687, changes in drop velocity of multidroplet drops can be regulated by modifying the amplitude of the electrical drive signal pulses. Compared to the drive method disclosed in EP 0 968 822 this approach requires electronic drive circuitry allowing the voltage amplitude to vary between individual drive pulses in a sequence of drive pulses generating a multidroplet drop. This requirement adds costs and complexity to the print head drive electronics.
In summary, some prior art regarding equalizing the velocity of multidroplet drops from a multipulse grayscale print head focus on tailored waveforms to increase the velocity of the single-droplet drop relative to that of the multi-droplet drops, or to reduce the velocity of multi-droplet drops relative to that of the single-droplet drop. Other prior art focuses on tailored electronic drive circuitry to adjust voltage amplitude of the applied waveforms. They share the same objective of reducing drop velocity variations, thereby also reducing dot placement errors on a receiving medium. They also share the same disadvantage in that these approaches are not open to end users or system integrators of multipulse grayscale print heads, i.e. drive waveforms and print head driver electronics are usually proprietary to the print head manufacturer. Nonetheless, printing systems for industrial printing applications, combining high speed with high quality, and thus requiring high drop velocity uniformity, are developed by system integrators in close cooperation with end users. A need exist to equalize drop velocity in multipulse grayscale inkjet printing applications in a manner that is open to system integrators or end users.